Page 272 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Electrode Polarisations 249

9.4.2 Anodic Activation Polarisation
The basic concepts of composite or single-phase MIEC electrodes are equally
appIicable to anodes. Traditionally, however, the typicaI anode used to date has
been a composite mixture of Ni and YSZ. The presence of YSZ not only suppresses
the thermally induced coarsening of Ni, but it also introduces MIEC
characteristics. Other anodes currently under investigation are based on cermets
of copper, which are being explored for direct oxidation of hydrocarbon fuels
[39]. These types of anodes are in an early stage of development and thus their
polarisation behavior is not discussed here. In so far as single-phase anodes are
concerned, some work has been reported in the literature, most notably on La-
SrTiOs [40, 411. Work on this as well as other perovskite-based anodes is in its
infancy, and is not elaborated upon further. The discussion in this chapter is
confined to Ni + YSZ cermet anodes.
Although the basic concepts of anode reaction are similar to the cathode, the
details may be different, and are not well understood at the present time.
The overall anodic reaction may be given by:
02-(electrolyte) + Hz(fue1 gas) -+ HzO(fue1 gas) -t 2e’(anode)

One of the scenarios could involve the following steps:

Adsorption of H2 on the surface of YSZ or Ni from the anode
&(fuel gas) 3 H2,,(YSZ or Ni)

Surface diffusion of adsorbed H2 to TPB
HZads (YSZ or Ni) ---f H2& (TPB)

Anodic electrochemical reaction
@(electrolyte) + Hz,,,,(TPB) + HzO (fuel gas) + 2e’(anode)
+ Vy(electro1yte)

In the preceding, the Kroger-Vink notation has been used. Similar to the
cathodic overpotential, the anodic activation overpotential also depends upon
material properties, microstructure, atmosphere, temperature and current
density: that is,
= f(materia1 properties, microstructure, temperature,
atmosphere, current density) (26)

Assuming a phenomenological model, anodic polarisation can be described
using the Butler-Volmer equation, and its Iow current density (linear) and high
current density (Tafel) limits. Experimental results for some selected cases can be